Light incident on subwavelength metallic structures can set up collective oscillations of the materials' conduction electrons, termed localized surface plasmon resonances. The attributes of such resonances depend on a number of factors, including the material, size, shape, and orientation of the plasmonic structure. The strong interaction and localization of light in plasmonic structures make them attractive candidates for controlling the properties of light, including intensity, phase, polarization, direction, and spectral power distribution. In the visible spectrum, these effects can be employed for the generation and modification of color (see ref. 1). However, a large portion of prior demonstrations of plasmonic structures have shown properties that are static in time, limiting their application for real-time light modulation. Therefore, a need exists for mechanisms by which plasmonic structures can modulate light in a dynamic and controllable manner, and preferably with a fast switching time.
Application of an external electric field offers a potential means to modulate the optical properties of matter, including by imparting alignment to anisotropic materials. In general, the permanent or induced dipole moment and resulting polarizability of a molecule is too small to couple to external electric fields to overcome disordering thermal forces, preventing alignment. If anisotropic molecules are condensed into a liquid crystal phase, then the additional van der Waal forces from the near-neighbor interactions increase the polarizability to enable alignment of the molecules and control the optical properties. The electric-field-induced alignment of anisotropic molecules in liquid crystal phases has enabled disruptive technologies such as smart phones and flat screen displays.
The switching time of these materials depends on the sum of their on- and off-times. The on-time needed to align the molecules into the direction of the applied electric field is predominately set by the magnitude of the field applied, τon≈γ/εE2, where γ is the viscosity, ε is the dielectric permittivity and E is the electric field. The off-time is related to the thermal rotational diffusion of the liquid crystal molecules and typically is the limiting factor to determine the overall switching time. In the case of liquid crystals, the near-neighbor interactions create strong electrohydrodynamic coupling, leading to a slow characteristic off-time, τoff≈γd2/K≈ms, where d is the cell thickness and K is the elastic constant of the liquid crystal. This well-known limitation has constrained potential electro-optic applications for decades.
A need exists for improved switching times.